Piezoelectric unit and method of making same



J, H. RAM

ec. i@ NIM,

PIEZOELECTRIC UNIT AND METHOD OF MAKING SAME',A

Filed July 8, 1940 INVENTOR -Jorm H-lZa-AM- BY ATTORNEY L a aannam' ivm.

Patented Dec. 16, 1941 PIEZOELECTRIC UNIT AND METHOD OF MAKING SAME,John H. Ream, Cleveland, Ohio, assigner to The Brush DevelopmentCompany, Cleveland, Ohio,

a corporation of- Ohio Application July 8, 1940, Serial No. v344,325

r19 fClaims.

`This invention relates in general to multipleplate `piezoelectriccrystal units and to an 'improved method for electroding suchmultipleplate units.

It is an object of this invention to provide .a piezoelectric unit thatmaybe madeinrelatively large quantities at low individual cost and withuniform individual characteristics.

It is another object to provide a simplied method for making suchpiezoelectric units in large quantitiesat 10W individual cost.

Another object of the .invention is to provide a process for Amaking atlow cost multiple-plate piezoelectric unitsof Rochelle salt whichutilize two different kindsof electrodes.

Another object of the invention isfto provide a simplified process forapplying to a Vpiezo/electric unit outer electrodes which aidmoisture-proofing -the unit.

Another object is to provide a multiple-plate piezoelectric unit havinga metallic sheath or jacket secured -to and covering substantially the.entire outersurface of the unit.

One of ,the principal diiculties involved in the manufacture of.multiple-plate Rochelle salt units for use in various typesofgpiezoelectric devices is that of producingpiezoelectric units whichcanbe made in :large quantities at 'low' cost. vAnother difficulty`is-to provide units of this kind which have uniform individualcharacteristics. Theindividual characteristics of units made by some ofthe methods of theipriorart'havebeen found to be non-uniform, thiscondition'lbeing apparently the result .of slight variations in the.intimacy of contact between the electrodes and the crystalline ipiezoelectric material, these slight -variations causingerather largeYvariationsin thecapacitance of'the units.

.Electrodeshave been appliedto Rochelle salt crystal assemblies "by two:methods `which have been used extensively-in commercial production. Onemethod, 'which -is described .in the U. S. Patent No. 1,995,257, granted@to C. B. Sawyer, consists of cementingelectrodes of .metal foil totheicrystal by'means of an adhesive that hasthe property of contractingupon drying, and rubbing the foil rinto intimate contact .withthecryStal before theadhesive has dried appreciably. While being athoroughly Vcommercial process, it is unduly expensive because asrheretofore practiced, it f involved many time-'consuming operations.The other method, whichiis 'described fully in U. S. Patent No.2,106,143, :granted `to VA, 'L W. Williams, consists of l.spraying thecrystal with al thin coating voffnely divided nconducting materal'suchas colloidal graphite, the material being applied in the wet state.Crystal units having graphite electrodes have a higher capacitance thansimilar units provided with foil electrodes because of the more intimateelectrode contact. Units provided with such electrodes also manifestgreater uniformity in their capacitance and sensitivity when produced inlarge quantities at commercial rates of production. This is due in partto the fact that by using a mask during the spraying operation it ispossible to control the size of the electrodes with greater accuracy.However, the method has its disadvantages, one of which lies in the factthat in order to attach connecting leads to the outer or remoteelectrodes and to increase the conductivity of the electrodes, a layerof metal foil needs to be cemented in contact with the sprayedelectrode. This involves one or more additional operations in theprocess and consequently adds to the individual cost of manufacture. Inaddition, units provided with the graphite electrodes exhibit a greatervariation in sensitivity and capacitance with temperature changes thando similar units provided with foil electrodes, and hence such units maybe objectionable for some uses.

It has been found that by modifying the first method in a novel manner,a simplified and improved method is obtained which involves fewertime-consuming operations and thereby permits material reductions inmanufacturing cost. Briefly, the new method is characterized by the useof outer foil electrodes which are wrapped about thepiezoelectricassembly in such manner as to overlap a suitably disposed electrodeextension, thereby effectively securing the extension in good mechanicaland electrical engagement With the assembly and with the foil sheath orjacket which is formed by the electrode Wrapping. It is through the useof this characterizing feature that the foregoing objects are achieved.

The invention and its modifications will be understood more fullythrough reference to the drawing, and to the following detailed'description.

In the drawing:

Fig. 1 is an enlarged and exploded perspective View of a 2ply bimorphassembly ready to be electroded in'accordance with the invention.

'-Figs. 2 and 3 'are plan views illustrating preferred method steps bywhich a foil electrode may be appliedto the inner face of eachpiezoelectric crystal plate of the type used in the bimorph assembly ofFig. 1.

Fig. lis a plan View illustrating the preferred method step by which oneouter foil electrode is applied in accordance with this invention to abimorph assembly of the type shown in Fig. 1.

Fig. 5 is an enlarged perspective view of a partially electroded bimorphassembly showing the electrode which has been applied thereto by themethod step of Fig. 4 and showing an electrode extension disposed inposition on the surface of the assembly in preparation for the followingmethod step.

Fig. 6 is a plan view illustrating the preferred method step by which asecond outer electrode of metal foil is applied to the unelectroded faceof the partially electroded bimorph assembly shown in Fig. 5.

Fig. 7 is an enlarged sectional view of the completed bimorph unit whichhas been made by the process illustrated by Figs. 1 through 6, thesection being taken at a position corresponding approximately to thatindicated by line 1, 'I of Fig. 5.

Fig. 8 is an exploded perspective view of a bimorph Vassembly havinginner electrodes of sprayed graphite, and ready to have outer electrodesapplied in accordance With the method of this invention.

Fig. 9 is an enlarged sectional view of the completedA bimorph unit ofFig. 8 after it has been electroded Vexternally in accordance with theinvention, the section being taken approximately at the positionindicated by line 9, 9 of Fig. 8.

Fig. 10 is an enlarged sectional View of a modified form of theinvention and illustrates a piezoelectric unit having a sheath electrodeformed from a single length of foil.

Referring now to Fig. 1 which shows the construction of a -simple 2-plyassembly ready to be electroded in accordance with this invention, itwill be seen that the assembly l contains plates 2, 2 of crystallinepiezoelectric material which have electrodes 3, 3 composed of metallicfoil, such as tin foil, secured to their inner faces. The plates aresecured together in any suitable manner as by means of cement and areprovided with a lead or electrode extension 4 positioned between theirinner faces in electrical engagement with the inner electrodes. In anassembly as thus-described, theelectrodes 3, 3 should be spaced inwardlyfrom all'of the edges of the face to which each is applied so as toprovide a moderate marginall around. It will be understood from thesubsequent description that a margin is needed to prevent electricalleakage between the inner and outer electrodes.

In making a unit of the kind shown in Fig. 1, the individualpiezoelectric plates 2, 2 are first electroded in any suitable manner.When foil electrodes are to` be applied, a procedure such asV thatillustrated by Figs. 2 and 3 is preferred.

In' this process, a strip of foil 5 having one sur face coated with asuitable adhesive is stretched out upon a fiat surface and individualplates of piezoelectric material are then applied one by one or ingroups to the adhesively coated face. While the strip of foil may bewider than the unit. preferably it is cut to a predetermined width, suchthat when the individual plates are applied to it, substantially in therelation shown, eoual margins will be formed adjacent the top and bottomedges. When the plates are trapezoidal in shape, they preferably areapplied in the alternate arrangement shown in the figure, so as tofacilitate subsequent handling. After a suitable number of such plateshas been appliedto the adhesive face of the strip of foil, the platesand the adhering foil are turned over vtrical contact with theelectrode.

so that the foil is uppermost as shown in Fig. 3. The foil is nextrubbed into intimate contact with each plate, after which each plate istrimmed by cutting through the foil substantially on the dot-and-dashlines A-A, B-B'. The excess' foil is then peeled off, leaving marginsadjacent the trimmed edges of leach plate. In this way, a foil electrodemay be applied to a face of a plate by a process involving a minimumnumber of operations. After the inner electrode has been applied, a pairof the semi-electroded plates,

having proper mutual piezoelectric orientation whenplaced together withtheir electroded faces in contact, are selected. An electrode extension4 is preferably applied to the electroded face of one of the selectedplates, the extension being applied thereto yin such manner as to makeelec- Suitable cement may then be applied to either or both of theelectroded faces of the selected plates, after which the plates arejuxtaposed and firmly pressed together for the purpose of bringing theelectrodes 3, 3 into contact with the lead extension 4 and for thefurther purpose of pressing any excess cement out from between the twoplates. A Z-ply assembly which is ready to be electroded inaccordancewith the invention is thereby produced.

'I'he rst step of the novel electroding process of the invention isillustrated by Fig. 4. In performing this step of the process, a stripof foil 6 having suitable adhesive applied to one face thereof isstretched out. upon a flat surface with the adhesive face uppermost.Two-ply assemblies, such as the one shown in Fig. 1, are then placedupon the foil strip, being positioned thereon preferably so as to leavea margin adjacent to the edge of the assembly from which electrodeextension 4 projects. When the assemblies are applied to the foil, theyshould also be spaced apart an appropriate distance for a purpose whichWill be understood from the subsequent description. After a suitablenumber have been so applied, the foil is cut substantially-on the linesC-C, C-C, thus freeing each assembly from the others. It will be notedthat by cutting the foil on these lines, wide borders 8, 9, I 0 of foilremain adjacent three edges of each assembly. The next step in theprocess consists in folding the borders preferably in the order 9, 8,I0, and wrapping them about the-sides of the assembly and onto theopposite face, as shown in Fig. 5.

After one outer electrode has been applied in the manner last describedand its border portions have been folded about the edges to overlap onto the opposite face, an electrode extension II is then applied to theouter surface of the assembly as thus partially electroded, theextension preferably being disposed in contact with one of the borderportions. It may be held in place temporarily by a small amount ofadhesive. 5 shows the partially electroded assembly having a leadextension in position on an exposed crystalline face of the assembly.The partially electroded assembly as thus produced is then ready forrthe final step in the electroding process.

The final step is illustrated by Fig. 6. It will be apparent at theoutset that this step of the process is substantially a repetition ofthe step illustrated by'Fig. 4 except that an assembly such as the oneshown in Fig. 5 is here involved. In this step, the partially electrodesassemblies are applied to a strip l2 of adhesively coated foil, beingplaced thereon with thepreviously electroded face of the unit uppermostand with the unelectroded face which carries electrode extension II, incontact with the adhesive layer of the electrode strip. A convenientnumber of such assemblies is applied to the strip after which theelectroding strip is cut substantially on the lines D-D', D-D so as toseparate the assemblies from each other. Again the border portions I3,I4 and I5 of the foil strip are folded about the edges of the assemblyand on to the opposite exterior face of the unit, thereby completing thestep. It will be noted that by reason of the fact that the two outerelectrodes applied by the foregoing process steps mutually overlap onthree edges of the unit, a sheath or jacket of metal foil is formedwhich is secured to substantially the entire exposed surface of theassembly. This metal sheath, being relatively impervious to moisture,aids in preventing deterioration of the unit as well as providing thenecessary'electrodes. In addition the sheath facilitates the subsequentapplication of a smooth coating of suitable moisture-proong materialsuch as shellac.

The nature of this moisture-proofing sheath will be more apparent fromthe sectional View shown in Fig. 7. It will be seen that each of theexterior faces of the piezoelectric plates is in contact oversubstantially its whole area with the electrically conductive metalfoil. Moreover, it will be observed that each of the enclosed edges ofthe unit is protected by means of a double layer of foil. The doublelayer on the edges and corners protects the parts of the unit which aremost easily injured, and also permits a more uniform coating ofmoisture-prooiing material to be applied to the parts when suchprotective material is used. The metal sheath is particularly beneficialwhen the units are moisture-proofed by being dipped into materials suchas shellac, or the polymerized vinyl chloride compositions known asKorolac, since it provides a smooth metallic base to which a smooth,uniform layer of such materials will adhere tenaciously. It will beunderstood that the electrodes and lead extensions shown in Fig. 7 aregreatly exaggerated in thickness.

It will be noted that these various advantages have been obtainedthrough the use of a process which involves a minimum number ofoperations. By reason of this latter fact, the individual cost of theunits is materially reduced as compared with the cost of units made byprior methods. In addition, since only the interior electrodes of theassemblies are trimmed, less variation occurs in the electroded arms ofthe various units than occurs in units in which both the inner and outerelectrodes are trimmed. Hence, the capacitance of units made by thisprocess is more uniform. This latter advantage may be utilized even moreeffectively by substituting sprayed graphite electrodes or electrodescomposed of similar finely conductive material for the inner foilelectrodes 3, 3 since when such sprayed electrodes are applied to theunits by means of masks, the variation in electroded area is reduced toa minimum. Such units also possess additional advantages which will bediscussed more fully below.

Fig. 8 illustrates a 2-ply piezoelectric assembly which has innerelectrodes I6, I6 composed of sprayed colloidal graphite with a foilelectrode extension I'I in electrical contact therewith. The method bywhich such multiple assemblies may be made is fully disclosed inWilliams Patent No.

2,106,143. Such an assembly may be electroded externally with foilelectrodes applied in accordance with this invention by following thesteps described above and illustrated by Figs. 4, 5 and 6. In accordancewith a preferred modification of the method, the outer electrodeextension I8 is disposed in contact with one of the folded borderportions of the outer foil electrode first applied to the assembly. Theunit produced by the process and embodying this latter modification, hasa structure like that shown in section in Fig. 9.

As indicated above, units having inner electrodes of graphite or similarfinely-divided conductive material, and outer foil electrodes applied inaccordance with this invention to form an enclosing sheath, haveadditional advantages which enhance their utility as compared withsimilar units having inner electrodes of foil. Such units accordinglyare a preferred form of the invention. The advantages here referred toinclude a higher degree of uniformity in capacitance, greatercapacitance, and lower individual cost. In addition, such units are animprovement upon the all-graphite units of the above mentioned WilliamsPatent since they are cheaper to make and exhibit somewhat lessvariation in sensitivity and capacitance with temperature. As previouslypointed out, this latter variation in the all-graphite units may bequite objectionable under some conditions of use, but since thevariation may be materially reduced by substituting the outer sheathelectrodes of this invention for the outer graphite electrodes of theWilliams units, the objectionable feature is somewhat mitigated.

It will be observed that when assemblies are electroded in accordancewith the method of this invention, the lead extension is preferablydisposed between two overlapped portions of foil. As previously pointedout, this disposition insures good electrical Contact between theextension and the sheet electrode, and in addition, results in a strongmechanical joint. While the extension may be disposed elsewhere inaccordance with the invention, as for example in direct contact with thepiezoelectric material, its placement between the laminations of thelapped joint is somewhat advantageous especially when roughened leadssuch as shown and described in Williams Patent No. 2,106,143v are usedor when the leads are of moderately heavy gauge material. It will beappreciated that under these conditions, it is likely to be moredifficult to rub the lead into intimate and direct contact with thepiezoelectric crystalline material. with the consequent result thatsmall areas of the latter material will not be effectively electroded.This condition is avoided, as will be apparent, when foil electrodematerial is placed between the lead and the piezoelectric material.

'I'he advantages of units made in accordance with this invention,insofar as cost is concerned, will be understood more fully by comparingthe steps involved in the method of this invention with the stepsinvolved in making a similar unit by the process described in WilliamsPatent No. 2,106,143 and by the process described in Sawyer Patent No.1,995,257. For the purpose of facilitating this comparison the principalsteps involved in making a 2-ply unit in accordance with the process ofthe latter patent, wherein all electrodes applied to the plates arecomposed of metallic foil, are here outlined:

1. Provide two crystalline plates.

. Apply adhesive-coated foil to one face of each plate leaving marginsadjacent the end edges.

3. Trim the two side edges of the foil to 'complete a margin surroundingthe electrode.

4. Apply cement to an electroded face of'one plate.

5. Bring electroded faces of the two plates together with an electrodeextension between and squeeze plates together to exude excess cement. f

6. Apply adhesive-coated foil to exterior faces of assembly.

7. Position electrode extension in contact with one or both exteriorfoil electrodes.

8. Apply adhesive-coated foil patch over each electrode extension tohold the extension in place.

9. Trim marginal edges of exterior electrodes and the patches aixedthereto.

In making a 2-ply bimorph in accordance with the method disclosedY inWilliams Patent No. 2,106,143, the principal steps would besubstantially as follows:

(Note: Each step of this method which corresponds with a similar step inthe above list bears the same numeral.)

1. Provide two crystallineA plates.

2. Spraycolloidal graphite on one face of each plate using a mask.

4. Apply cement to a graphited face of one plate.

5. Bring graphited faces of both plates together with electrodeextension between them and squeeze the plates together to exude excesscement. Y

6. Spray colloidal graphite on the remote faces faces using a mask.

7. Position lead or leads in contact with one or both graphiteelectrodes.

8. Apply adhesive-coated foil patch over each lead.

9. Trim margins of patches to correspond with margins of outer graphiteelectrodes.

' A bimorph made by this method will have the general appearance of theunit shown in Figs. 12 or 13 of the Williams patent.

In making a 2-ply bimorph in accordance with the most preferred form ofthe invention disclosed herein, the principal steps would besubstantially as follows:

1. Provide two crystalline plates.

2. Apply electrode of finely divided conductive material to one face ofeach plate using mask (by spraying, sputtering or otherwise).

4. Apply cement to electroded face of one plate.

5. Bring electroded faces of both plates together with electrodeextension between and squeeze the plates together to exude excesscement.

6. Apply adhesive-coated foil to one exterior face of the assemblycutting the foil, if necessary, to provide a margin on the lead edge andto provide projecting borders on the remaining edges. l

Fold borders about the assembly in the manner described in connectionwith Figs. 4 and 5.

7. Apply adhesive-coated lead to border portion of the electrode. y

B. Repeat steps 6 and A on the opposite exterior face of the assembly.

In comparing these three methods, it will be seen that the method ofthis invention avoids steps 3 and 9 of the Sawyer process and avoidsstep 9 of the Williams process. As will be noted, these are the trimmingsteps of each process. Herein lies the particular feature of the methodof this invention insofar as cost reduction is concerned, since thesetrimming steps acoount for at least 8 individual operations in theSawyer process and 6 in the Williams process, each of which requires`special care and skill and is timeconsuming. It will be observed thatsteps A and B of the method of this invention replace the trimming stepsand substitute steps therefor which may be performed quickly and easilyand with a minimum of care consistent with quality. The expensive stepsof the prior methods are thereby replaced with steps which permit ahigher rate of production with a consequent reduction in the individualcost of the units.

The invention has been explained through reference to certain specificforms which should be considered as illustrative in character ratherthan limiting, since various modifications may be made. For example,while the invention has been described in connection with two-plyassemblies, it will be obvious that assemblies composed of more than twoindividual piezoelectric plates may be electroded externally in the samemanner. It will also be understood that while the preferred form of theexternal electrode consists of two sheets of foil joined together inside by side relationship to form the encircling electrode previouslydescribed, such an encircling electrode may lbe made from asingle lengthof foil as shown in Fig. 10, or from a plurality of lengths, opposedmargins of which are overlapped and joined together to form a lappedjoint. A further embodiment of the invention may consist of apiezoelectric unit which includes an encircling jacket or sheathelectrode with or without an electrode extension secured to it betweenthe overlapping laminations of a lapped joint in the electrode, and suchjacket or sheath electrode may function as the sole external electrode,or it may be used in conjunction with auxiliary outer electrodes such aselectrodes composed of colloidal graphite. In such a unit, the foiljacket functions to protect the unit from moisture and damage asdescribed above, and retains its advantages over similar protectivejackets which are more diicult and therefore more expensive to apply.Numerous other modifications may be made and will be apparent to thoseskilled in the art. Accordingly the scope of the invention should bedetermined from the following claims taken in conjunction with theforegoing description and explanation.

What I claim is:

1. As an article of manufacture, an operative multiple-platepiezoelectric unit having a pair of remote major faces and interveningside and end faces, said unit being provided with an encircling jacketof metal foil secured to said major faces and said intervening sidefaces, each of the edges of the unit which underlie and engage the saidjacket being covered by a lapped joint in said jacket consisting of atleast two layers of overlapping metal foil.

2. As an article of manufacture, an operative multiple-platepiezoelectric unit having a pair of remote major faces and interveningside and end faces; an encircling jacket electrode of metal foilcomposed of two quadrilateral sections or" foil joined together in sideby side relationship, the rst of said sections covering and beingsecured in intimate relationship to substantially all of a first of saidvmajor surfaces and extending onto adjacent side faces of the unit andonto at least a part of the second major face, the second ofsaidsections covering and being secured in intimate relationship tosubstantially all of the uncovered portion of the second major face, andto at least the opposedmarginal portions of the first section of foil toform lapped joints therewith.

3. An article as claimed in claim 2 wherein the said second sectionoverlaps the said opposed marginal portions of the first section of foiland its opposed margins extend onto at least a part of the oppositemajor face, thereby providing two layers of foil on each of theencircled edges of the unit.

4. An article as claimed in claim 2 wherein an electrode extension isdisposed underneath and in immediate electrical contact with at leastone of said sections of foil and wherein said electrode extensionprojects beyond a bounding edge of the unit.

5. As an article of manufacture, an operative multiple-platepiezoelectricunit having an outer encircling jacket electrode of metalfoil,V said jacket having at least one lapped jointI therein and anelectrode extension disposed between'and secured to the overlappingportions of foil which form the said lapped joint.

6.1An article of manufacture as claimed in claim 5 wherein said jacketis composed of av single section of foil, the opposed marginal por`tions of which form the lapped joint.

7. An article as claimed in claim 5 whereinY saideelectrode is composedcfa plurality of sections of foil which are joined together in side byside relationship, each of the joints rbeing lapped joints.

8. An article as claimed in claim 5 wherein said piezoelectric unit is amultiple-plate unit having a pair of remote major faces and interveningside and end faces, and wherein said jacket is composed of twoquadrilateral sections of metal foil joined together in side by siderelationship, the iirst of said sections covering and being secured inintimate relationship to substantially all of one of said major surfacesand extending on to adjacent side faces of the unit and on to at least apart of the opposite major face, the second of said sections covering,and being secured in intimate relationship to, substantially all of theuncovered portion of the latter major face and to opposed marginalportions of the first of said sections of foil.

9. An article as claimed in claim 5 wherein the jacket electrode isfolded about and secured to an end face of the unit, thereby forming asheath electrode.

10. An article as claimed in claim 5 wherein electrodes applied to theinner faces of the piezoelectric plates composing the unit are composedof finely divided electrically conductive material.

11. As an article of manufacture, a multipleplate piezoelectric unitcomprising: a plurality of juxtaposed plates secured together flatwiseand appropriately oriented to provide an operative assembly; an innerelectrode system providing electrodes at inner faces of said plates andincluding an electrode extension which projects beyond a bounding faceof the unit; an outer electrode system effectively insulated from thesaid inner system and providing electrodes for the outer faces of theoutermost plates of the unit, said outer system including a sheathelectrode of metal foil which is secured to said outer faces and tointervening edges of the plates of the unit` and which has a lappedjoint therein formed of mutually overlapped portions of metal foil whichareseCured' to each other and to an electrode extension which isdisposed between the overlapped portions of the joint and projectstherefrom.V l y 12. An article as claimed in claim 1l wherein theelectrodes ofA said system consist of finely divided electricallyconductive material.

13. An article as claimed in claim 11 wherein saidsheath electrode isopen at only one end.

14. An article as claimed invclaim ll wherein said sheath electrode iscomposed of two sections of foil secured together in side by siderelationship by means of lapped joints.

15. A piezoelectric unit comprising a plurality of quadrilateral'platesof crystalline piezoelectric material secured together flatwise in faceto face relationship, an inner electrode' system providing electrodesfor the inner faces of the plates and comprising electrodes composed offinely divided electrically conductive material secured to the innerfaces of each of said plates', and a pair of metallic foil electrodessecured one to each of the exteriorV faces ofthe outermost plates of theunit, said pair of metallic foil electrodes having border portionsthereof folded about three contiguous edge faces of the unit in mutuallyoverlapping relationship and secured to each other and to said edgefaces to form a moisture-proofing sheath covering substantially all ofsaid unit except one edge face thereof, an electrode extension securedin electrical engagement with an innerV electrode and extending out ofsaid unit at thesaid uncovered edge thereof, and an electrode extensionsecured to the foilelectrodes betweenelaminations of the overlappedborder portions.

16. In the method of making a multiple-plate piezoelectric unit, thesteps of: providing an assembly comprising a plurality of juxtaposedpiezoelectric plates secured together flatwise and including an innerelectrode system providing electrodes for inner faces of said juxtaposedplates, said electrode system including an electrode extension whichprojects from an edge of the assembly; intimately securing tosubstantially all of the outer face of one of the outermost plates ofthe assembly a sheet of metallic foil electrode material having lateralborder portions which extend beyond and along opposite edges of saidface and which terminate adjacent the said edge beyond which the innerelectrode extension projects; folding said lateral border portions ofsaid sheet onto the contiguous edge faces of the assembly and onto theopposite outer face of the assembly; thereafter securing a second sheetof metallic foil to the said opposite face of the assembly to overlapsaid folded border portions thereby forming an open ended encirclingelectrode.

17. In the method of making a multiple-plate piezoelectric unit, thesteps of: providing an assembly comprising a plurality of juxtaposedpiezoelectric plates secured together iiatwise and including an innerelectrode system providing electrodes for inner faces` of saidjuxtaposed plates, said electrode system including an electrodeextension which projects from an edge of the assembly; intimatelysecuring to substantially all of the outer face of one of the outermostplates of the assembly a sheet of metallic foil electrode materialhaving lateral border por- 6 aaecse tions which eXtend beyond and alongopposite edges of said face and which terminate adjacent the said edgebeyond which the inner electrode extension projects; folding saidlateral border portions of said "sheet onto the contiguous edge faces ofthe assembly and onto the opposite outer face of the assembly; aixing anouter electrode extension to the outer surface of said unit as thuspartially electroded in such manner as to project beyond the same edgefrom which the inner electrode extension projects; thereafter securing asecond sheet of metallic foil to the said opposite face of the assemblyto overlap Vsaid folded border portions and the said outer electrodeextension, thereby forming an open ended encircling electrode having anouter electrode extension projecting therefrom.

18. In the method of making a multiple-plate piezoelectric unit, thesteps of providing an assembly` comprising a plurality of juxtaposedpiezoelectric plates secured together atwise and including an innerelectrode system providing electrodes for inner surfaces of saidjuxtaposed plates, said electrode system including an electrodeextension which projects from an edge of the assembly; intimatelysecuring to substantially all of the outer face of one of the outermostplat-es of the assembly a sheet of metallic foil electrode materialhaving lateral border portions which extend beyond and along oppositeedges of said face and which terminate adjacent thev said edge' beyondwhich the inner electrode ex-A tension projects; folding said lateralborder portions of said sheet onto the contiguous edge faces of theassembly and onto the opposite outer face of the assembly; aiXing anouter electrode eX- tension in contact with one of said folded borderportions in such manner as to project beyond the lsame edge from whichthe inner'electrode assembly comprising a plurality of juxtaposedpiezoelectric plates secured together flatwise and including Van innerelectrode system providing electrodes for inner surfaces of saidjuxtaposed plates, said electrode system including an electrodeextension which projects from an edge of the assembly; intimatelysecuring to substantially all of the outer face of one of the outermostplates of the assembly, a sheet of metallic foil electrode materialhaving border portions .I which extend beyond all of the edges of saidface except the said edge beyond which the inner electrode extensionprojects; folding said border portions of said sheet onto the contiguousedge faces of the assembly and onto the opposite outer face of theassembly; affixing an outer electrode extension to one of said foldedborder portions in such manner as to project beyond the same edge fromwhich the inner electrode extension projects; thereafter securing asecond sheet of @metallic foil to the said opposite face of the assemblyto overlap said folded border portions and the said outer electrodeextension, thereby formingran encircling electrode sheath having anelectrode extension projecting therefromfrom between the overlappinglaminations of a lapped joint therein. l

JOHN I-I. REAM.

